A device that extracts the energy of a chemical change as electrical energy is a cell (chemical cell). The process of using electricity to cause a chemical change is called electrolysis.
Basic Knowledge
A chemical cell combines two types of metals with an electrolyte solution and extracts electrical energy from a chemical change (an oxidation-reduction reaction).
Representative cells:
① Voltaic cell (the original form of a cell. Zinc plate, copper plate, dilute sulfuric acid)
Zinc plate: Zn → Zn2+ + 2e- (oxidation・negative electrode(-) )
Copper plate: 2H+ + 2e- → H2 (reduction・positive electrode(+)）
② Manganese dry cell (a representative dry cell. Widely used in daily products)
③ Lead-acid battery (a rechargeable secondary cell. An automobile battery)
④ Fuel cell: Extracts electricity from the reaction between hydrogen and oxygen. 2H2 + O2 → 2H2O (environmentally friendly because the only emission is water)
Electrolyte: A substance that dissolves in water and conducts electricity (sodium chloride, sulfuric acid, hydrochloric acid, and so on)
Nonelectrolyte: A substance that does not conduct electricity even when dissolved in water (sugar, ethanol, and so on)
Chemical cell (a device that extracts electrical energy from a chemical change)
Electrolyte (a substance that dissolves in water, separates into ions, and conducts electricity)
Voltaic cell (the first cell, made of a zinc plate, copper plate, and dilute sulfuric acid)
Fuel cell (extracts electricity by reacting hydrogen with oxygen. The emission is water)
Primary cell (a nonrechargeable cell. Dry cells and alkaline cells)
Secondary cell (a rechargeable cell that can be used repeatedly. Lead-acid batteries and lithium-ion batteries)
Further Study
Oxidation occurs at the negative electrode of a cell, and reduction occurs at the positive electrode. A metal with a greater tendency to ionize becomes the negative electrode and dissolves (it is oxidized). The order of ionization tendency (large → small): K > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > (H2) > Cu > Hg > Ag > Pt > Au
In a voltaic cell, Zn has a greater ionization tendency than Cu, so Zn dissolves as the negative electrode.
Fuel cells are an important technology for realizing a hydrogen-energy society. Hydrogen can be burned to produce heat or converted directly into electricity in a fuel cell. Electrical conversion is more efficient, and the fact that it does not emit CO2 is also a major advantage.
- Chemical cell = conversion from chemical change to electrical energy
- Oxidation occurs at the negative electrode(−), and reduction occurs at the positive electrode(+)
- The metal with the greater ionization tendency becomes the negative electrode
- The only emission from a fuel cell is water (clean energy)
- Electrolyte = dissolves in water and conducts electricity (NaCl・HCl・NaOH)
- Nonelectrolyte = does not conduct electricity even when dissolved in water (sugar・ethanol)
- Secondary cell = rechargeable (lead-acid battery・lithium-ion battery)
Points to Note
① The relationship that the negative electrode(−) is oxidized and the positive electrode(+) is reduced can be confusing. Electrons flow from the negative electrode to the positive electrode through the external circuit. ② Distinguish between a primary cell (disposable) and a secondary cell (rechargeable). ③ An electrolyte is a substance that separates into ions in water, but a metal alone is not an electrolyte (although it conducts electricity). ④ A fuel cell does not burn fuel; it causes a reaction electrochemically.
Practice
- State the metals used for the negative and positive electrodes of a voltaic cell.
- Explain the difference between electrolytes and nonelectrolytes, giving examples.
- Explain the characteristics of a fuel cell from the viewpoints of its emissions and energy conversion.